1. a Department of Mathematical Statistics and Actuarial Science , University of the Free State , Bloemfontein, South Africa
2. b Quintiles Biostatistics , Bloemfontein, South Africa
3. c Department of Pharmacology , University of Toronto , Toronto, Ontario, Canada
4. d Clinical Biostatistics, Boehringer Ingelheim Pharma, Biberach an der Riss , Germany
5. e Institute for Biometry , University of Ulm , Ulm, Germany